We propose to investigate the interplay between convective flow (ventilation and circulation) on one hand and O2 uptake and CO2 output on the other, with specific emphasis on: 1) situations where a decrease in convective flow may limit O2 supply or CO2 elimination, 2) mechanisms by which O2 and CO2 exchange affect convective flow, and 3) interaction between ventilation and circulation. The five projects constituting this pogram deal with: 1) effects of gravity on the interaction between air and blood flow in the lungs, 2) effects of ventilation on cardiac output, 3) ventilatory response to CO2 load, 4) local circulation and possible limitations in tissue oxygenation during exposure to 100% O2, and 5) redistribution of circulation when the ability to carry O2 is decreased by hemodilution. Interaction between the investigators is ensured by: 1) the shared laboratories, equipment and technical assistance, 2) the fact that most of them collaborate on several component projects, and 3) weekly research progress conferences.